1. Field of the Invention
The present invention relates to the improvement of an electron tube cathode which is used for a TV cathode ray tube or the like.
2. Description of the Related Art
FIG. 3 shows an electron tube cathode which is used for a TV cathode ray tube or an image pick-up tube such as that described in, for example, Japanese Patent Publication No. 5417/1989. In FIG. 3, the reference numeral 1 represents a base composed of nickel as the main ingredient and further containing a trace amount of reducing element such as silicon (Si) and magnesium (Mg); 2 represents a cathode sleeve composed of nichrome or the like; 5 represents an emissive material layer which is formed on the upper surface of the base 1. The emissive material layer is composed of alkaline earth metal oxides as the main ingredients and 0.1 to 20 wt % of a rare earth metal oxide such as scandium oxide. Further, the alkali earth metal oxides contain at least barium oxide and further strontium and/or calcium oxide. Finally, 3 in FIG. 3 represents a heater disposed in the base 1, for heating the cathode so as to emit thermions from the emissive material layer 5.
A method of forming the emissive material layer 5 on the base 1 in an electron tube cathode having the above-described structure will now be explained. Barium carbonate, strontium carbonate, calcium carbonate and a predetermined amount of scandium oxide are first mixed together with a binder and a solvent to prepare a suspension. The suspension is sprayed onto the base 1 to a thickness of about 800 .mu.m and thereafter heated by the heater 3 during the cathode ray tube evacuating process. At this time, the carbonates of the alkali earth metals are converted into alkali earth metal oxides. Thereafter, a part of the alkali earth metal oxides are reduced and activated so as to have semiconductivity. Thus, the emissive material layer 5 composed of a mixture of the alkali earth metal oxides and a rare earth metal oxide is formed on the base 1.
A part of the alkali earth metal oxides are reacted in the following manner in the activating process. The reducing elements such as silicon and magnesium, which are contained in the base 1, move to the interface between the alkali earth metal oxides and the base 1 by diffusion and react with the alkali earth metal oxides. For example, if the alkali earth metal oxide is assumed to be barium oxide (BaO), the reducing elements react in accordance with the following reaction formulas: EQU BaO+1/2Si=Ba+1/2Ba.sub.2 SiO.sub.4 ( 1) EQU BaO+MgO=Ba+MgO (2)
As a result of these reactions, a part of the alkali earth metal oxides which are formed on the base 1 are reduced to be an oxygen deficient semiconductor, thereby facilitating electron emission. If the emissive material layer contains no rare earth metal oxide, the operation is possible at a temperature of 700.degree. C. to 800.degree. C. and a current density of 0.5 to 0.8 A/cm.sup.2. If the emissive material layer contains a rare earth metal oxide, the operation is possible at a current density of 1.32 to 2.64 A/cm.sup.2.
Since the electron emission capability of an oxide cathode generally depends on the excess Ba content existing in the oxide, if no rare earth metal oxide is contained, the supply of excess Ba sufficient for the operation at a high current is not procured and the current density which enables the operation is low. In this case, excess Ba is not supplied sufficiently because the by-products of the above reactions such as magnesium oxide (MgO) and barium silicate (Ba.sub.2 SiO.sub.2) are concentrated on the grain boundary of the nickel of the base 1 or of the interface between the base 1 and the emissive material layer 5 to form what is called an intermediate layer. Thus, the rates of the reactions represented by the formulas (1) and (2) are controlled by the diffusion rates of the magnesium and silicon in the intermediate layer. On the other hand, if the emissive material layer contains a rare earth metal oxide, for example, scandium oxide (Sc.sub.2 O.sub.3), a part of reducing agent which diffuses and moves in the base 1 during the operation of the cathode reacts with scandium oxide (Sc.sub.2 O.sub.3) in accordance with the reaction formula (3) in the interface between the base 1 and the emissive material layer 5, thereby producing a small amount of scandium in the form of a metal. Further, a part of the metal scandium is dissolved in the nickel in the base 1 in the form of a solid, and a part thereof exists in the interface. EQU 1/2Sc.sub.2 O.sub.3 +3/2Mg=Sc+3/2MgO (3)
It is considered that since the metal scandium, produced by the reaction represented by the formula (3), has an action of decomposing the intermediate layer which has been formed on the base 1 or on the grain boundary of the nickel of the base 1 in accordance with the formula (4), the supply of excess Ba is improved and the operation is possible at a higher current density than in the case of containing no rare earth metal oxide. EQU 1/2Ba.sub.2 SiO.sub.4 +4/3Sc=Ba+1/2Si+2/3Sc.sub.2 O.sub.3 ( 4)
Japanese Patent Laid-Open No. 91358/1977 discloses a technique of producing a direct-heated cathode by preparing a base of an Ni alloy which contains a high-melting metal such as W and Mo for increasing the mechanical strength and a reducing agent such as Al, Si and Zr and coating the surface of the base on which an emissive material layer is formed with a layer of an alloy such as Ni-W and Ni-Mo.
Japanese Patent Laid-Open No. 75128/1990 discloses a cathode composed of a nickel base metal, an oxide layer of an alkali earth metal containing barium oxide and formed on the nickel base metal, and a metal layer containing scandium and at least one element selected from the group consisting of platinum, iridium and rhodium and formed between the nickel base metal and the oxide layer.
In the electron tube cathodes having the above-described structures, although the rare earth metal oxide improves the supply of excess Ba, the excess Ba supplying rate is controlled by the diffusion rate of the reducing agent in the nickel of the base. Further, the life characteristics of the cathode are greatly deteriorated in the operation at a high current density such as that not less than 2A/cm.sup.2.
The technique disclosed in Japanese Patent Laid-Open No. 91358/1977 is aimed at ameliorating the thermal deformation of the base, which is the intrinsic problem of a direct-heated cathode for emitting thermions from the emissive material layer by utilizing the heat of the base itself which is heated by the application of a current, by coating the base with a layer of an alloy such as Ni-W and Ni-Mo. This technique does not enable the operation at a high current density.
In the cathode disclosed in Japanese Patent Laid-Open No. 75128/1990, since the metal layer on the base is composed of a metal having smaller reducibility than tungsten or molybdenum, it has almost no barium oxide reducing effect for enabling the operation at a high current density.